For example, JP-A-2006-56374 and JP-A-2007-145040 disclose methods for measurement of force applied to a steered wheel of a vehicle. According to JP-A-2006-56374, a vehicle steering apparatus includes an auxiliary steering unit, a going-straight determination unit, an applied force calculating unit, and a steering control unit. The auxiliary steering unit is provided to each end of a steering shaft, which is used for independently manipulating right and left steered wheels of a vehicle according to an operation of a steering device. The applied force calculating unit calculates a difference between values of force caused in accordance with a road surface and applied to both steered wheels. The going-straight determination unit determines whether the vehicle goes straight in a running state. The steering control unit operates the auxiliary steering unit so as to control steering of at least one of the steered wheels when the going-straight determination unit determines that the vehicle goes straight, and the difference between the values of force applied to the right and left steered wheels calculated by the applied force calculating unit is larger than a predetermined value. Thus, the vehicle steering apparatus controls the steering in accordance with the force applied to the steered wheels. The force applied to the steered wheels is obtained by detecting axial force applied to a member, which is movable independently from a rack bar such as a tie rod or an auxiliary rack bar mechanism.
JP-A-2007-145040 discloses an electric steering device, which includes a load detecting unit and a steering power control unit. The load detecting unit includes a steering mechanism, which is connected with right and left steered wheels for steering the steered wheels, an electric actuator, which supplies steering power to the steering mechanism, a right load sensor, which detects load applied from the right steered wheel to the steering mechanism in the direction of the load, a left load sensor, which detects load applied from the left steered wheel to the steering mechanism in the direction of the load. The steering power control unit actuates the electric actuator based on at least a detection signal of the two right and left load sensors. The electric steering device further includes a sensor malfunction detection unit for determining whether the load detecting unit causes a malfunction by comparing the detection signal of the left load sensor with the detection signal of the right load sensor. The electric steering device controls steering assist in accordance with the force applied to the steered wheels. The force applied to the steered wheels is obtained by detecting the axial force applied to the tie rod.
According to JP-A-2006-56374 and JP-A-2007-145040, the force applied to the steered wheels is obtained by detecting the force applied to a device such as the tie rod and the auxiliary rack bar mechanism in the axial direction. The axial direction of the device such as the tie rod changes according to a steering state of the steered wheel. Here, self aligning torque (SAT) applied to each steered wheel may be detected so as to obtain a coefficient of friction of a road surface and the like. Specifically, a tire is applied with force (SAT) in the steering direction according to a slip angle and the like. The SAT changes in accordance with the slip angle of the tire, the coefficient of friction of the road surface, a running state, and the like. Therefore, the coefficient of friction of the road surface can be obtained by detecting the slip angle, the SAT, and the like. However, in the conventional method, in which the force applied to the device such as the a rod 93 in the axial direction is detected, the SAT may not be obtained with sufficient accuracy.